To assess the safety of the study drug by assessing solicited local reactions and systemic adverse events within 7 days after single booster dose. The local reactions and systemic adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 7 after administration of single booster dose of study vaccine];To assess the safety of the study drug by assessing Unsolicited Adverse Events. The unsolicited adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 21 after administration of single booster dose of study vaccine];To assess the immunogenicity of the study drug and the requirement for the MF59 adjuvant by a composite outcome: SARS-CoV-2-specific neutralizing antibody titers by live virus assay; spike protein and RBD-specific IgG antibody titers by multiplex assay.[Four blood draws on Day 0; Day 7; Days 28; Day 49 after administration of single booster dose of study vaccine]

To assess the safety of the study drug by assessing solicited local reactions and systemic adverse events within 7 days after single booster dose. The local reactions and systemic adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 7 after administration of single booster dose of study vaccine];To assess the safety of the study drug by assessing Unsolicited Adverse Events. The unsolicited adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 21 after administration of single booster dose of study vaccine];To assess the immunogenicity of the study drug and the requirement for the MF59 adjuvant by a composite outcome: SARS-CoV-2-specific neutralizing antibody titers by live virus assay; spike protein and RBD-specific IgG antibody titers by multiplex assay.[Four blood draws on Day 0; Day 7; Days 28; Day 49 after administration of single booster dose of study vaccine]

To assess the safety of the study drug by assessing solicited local reactions and systemic adverse events within 7 days after each dose.The local reactions and systemic adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 6 and from Day 28 to Day 34 after administration of first dose of study vaccine];To assess the safety of the study drug by assessing Unsolicited Adverse Events. The unsolicited adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 49 after administration of first dose of study vaccine];To assess the immunogenicity of the study drug and the requirement for the MF59 adjuvant by a composite outcome: SARS-CoV-2-specific neutralizing antibody titers by live virus assay; spike protein and RBD-specific IgG antibody titers by multiplex assay.[Four blood draws on Day 0; Day 7; Days 28; Day 49 after administration of first dose of study vaccine]

To assess the safety of the study drug by assessing solicited local reactions and systemic adverse events within 7 days after each dose.The local reactions and systemic adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 6 and from Day 28 to Day 34 after administration of first dose of study vaccine];To assess the safety of the study drug by assessing Unsolicited Adverse Events. The unsolicited adverse events will be assessed based on the United States FDA Guidance for Industry: Toxicity Grading Scale for Health Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials[Daily from Day 0 to Day 49 after administration of first dose of study vaccine];To assess the immunogenicity of the study drug and the requirement for the MF59 adjuvant by a composite outcome: SARS-CoV-2-specific neutralizing antibody titers by live virus assay; spike protein and RBD-specific IgG antibody titers by multiplex assay.[Four blood draws on Day 0; Day 7; Days 28; Day 49 after administration of first dose of study vaccine]